This invention relates to electric field proximity detection systems and particularly to electric field proximity circuits for touch sensitive or close proximity keyboards.
Many keyboards today are activated by depressing a keypad. A touch sensitive keyboard provides a similar I/O device, but is activated by merely touching rather than depressing each keypad. Touch sensitive keyboards are used in a variety of applications such as handheld computing devices (e.g., PDA), musical instruments, elevator switches, appliances, bank ATMs, and computers. Other types of touch sensitive keyboards can be activated by an object such as a finger or stylus touching the keypads displayed on a computer or terminal. A close proximity keyboard does not require contact, but is activated when an object (e.g., stylus or finger) is detected in close proximity to the keypad. Close proximity means the object is at a distance that should be detected for activation. The close proximity keyboard is especially useful where physical contact would result in undue wear or contamination in a sterile or hazardous environment.
In the past, touch sensitive and close proximity keyboards have used complicated detection systems. Some of these detection systems are described in U.S. Pat. Nos. 6,452,514 B1, 5,572,205, and 5,594,222, and U.S. published patent application Ser. No. 2002/0130848 A1. These patents and application describe use of two spaced apart electrodes or conductive pads such as a transmit electrode and a receiver electrode with a keypad. An object is detected as close to the keypad when it interferes with a signal coupled or transmitted between the two electrodes or pads. U.S. Pat. No. 3,971,013 takes a different approach using a gas panel to detect gas discharge due to contact to the gas panel. Due to the complexity of these detection designs, the costs impact the adoption rates of these types of keyboards.